The present invention relates to a non-aqueous electrolytic solution secondary battery, and in particular relates to a non-aqueous electrolytic solution secondary battery where an electrode winding group where a positive electrode formed by applying positive electrode active material including a lithium-manganese complex oxide comprising secondary particles formed of aggregated primary particles and conductive material on both surfaces of a strip-like collector by approximately even amounts to the both surfaces and a negative electrode from/in which lithium ions can be released/occluded through charging/discharging are wound through a separator is accommodated into a battery container having an internal pressure releasing mechanism which releases internal pressure at a predetermined internal pressure.
Because a non-aqueous electrolytic solution secondary battery represented by a lithium-ion secondary battery has a high energy density as its merit, it is mainly used as a power source or power supply for portable equipment such as a VTR camera, a notebook type personal computer, a portable or cellar telephone or the like. The interior structure of this battery is generally of a winding type as described below. Each of a positive electrode and a negative electrode of the electrodes of the battery is formed in a strip-shape where active material is applied to a metal foil, and a winding group is spirally formed by winding the positive electrode and the negative electrode through a separator so as not to come in direct contact with each other. This winding group is accommodated in a cylindrical battery container or can, and after the battery container is filled with electrolytic solution, it is sealed.
An ordinary cylindrical lithium-ion secondary battery has an external dimension of a diameter of 18 mm and a height of 65 mm, which is called 18650 type, and it is widely spread as a small-sized non-aqueous electrolytic solution secondary battery for a civilian use. Carbon material is ordinarily used as a negative electrode active material for the 18650 type lithium-ion secondary battery. The carbon material used may include graphite system material such as natural graphite, scale-shaped, aggregated artificial graphite, mesophase pitch system graphite, and amorphous carbon material prepared by sintering such furan resin as furfuryl alcohol or the like. On the other hand, as a positive electrode active material, one of lithium transition metallic complex oxides is used. Among the lithium transition metallic complex oxides, lithium cobaltate (LiCoO2) is widely used in view of balance of capacity, cycle characteristic or the like. Battery capacity of the 18650 type lithium-ion battery is approximately 1.3 Ah to 1.7 Ah and battery power (output) thereof is about 10 W or so.
Meanwhile, in order to cope with the environmental problems in the automotive industry, development of electric vehicles (EVs) whose power sources are confined completely to batteries so that there is no gas exhausting and development of hybrid electric vehicles (HEVs) where both internal combustion engines and batteries are used as their power sources have been facilitated and some of them have reached a practical state.
Secondary batteries for the EVs and HEVs are required to have high power and high-energy characteristics. Attention to the lithium-ion secondary batteries is being paid as secondary batteries which can meet this requirement. In order to spread the EVs and HEVs, it is necessary to reduce the prices of the secondary batteries. Low cost battery materials are required for achieving such price reduction. For example, regarding the positive electrode active material, special attention is paid to manganese oxides which are rich as resources, and improvement of such batteries has been made for high performance thereof. However, it is not considered that the power characteristic of the lithium-ion secondary battery using lithium-manganese complex oxide as the positive electrode active material is sufficient for the EVs and HEVs. In order to solve this problem, it has been studied that the area of the electrode is enlarged to achieve high capacity of the battery. However, such enlargement causes an increase in a battery size, which results in a practical difficulty in view of a mounting space for the battery in a vehicle.
Also, as the battery for the EVs and HEVs, not only high capacity but also high power which affect acceleration performance of a vehicle, namely reduction of the internal resistance of the battery, is required. In order to increase the reaction area of the electrode, this requirement can be met by utilizing a lithium-manganese complex oxide as the positive electrode active material. In particular, in order to increase a specific surface area, it is necessary to reduce the particle diameter of the lithium-manganese complex oxide. However, in a case of the small particle diameter, there occurs such a drawback that powder material is scattered during manufacture of an electrode or it is difficult to obtain appropriate slurry for applying the lithium-manganese complex oxide on both surfaces of a collector. It is possible to solve such a problem by utilizing a lithium-manganese complex oxide formed by secondary particles obtained by aggregating primary particles having a small particle diameter.
Moreover, in a case that a non-aqueous electrolytic solution secondary battery with a high power, as a contrivance for reducing reaction resistance between the positive electrode active material and the non-aqueous electrolytic solution, there has been known such a technique that the positive and negative electrodes are made thinner for reducing diffusion distance of lithium ions between the positive and negative electrodes or they are made longer for increasing the reaction areas of the positive and negative electrodes and so on. However, because large power is required in a case of a secondary battery for an electric vehicle, particularly for the HEV, the space in the battery is mainly occupied by portions other than the positive electrode active material so that the occupation volume of the positive electrode active material in the battery is reduced in the technique where the positive and negative electrodes is made thinner and/or longer. As a result, it is necessary to reduce the filling weight of the positive electrode active material. Such reduction means an increase in load per unit positive electrode active material. Because lithium ions are temporarily concentrated on a surface of the positive electrode active material during high rate charging/discharging cycle conducted by pulse-like current, particularly during discharging cycle, it becomes difficult to occlude lithium ions to a portion corresponding to a normal reaction side and load acting on the positive electrode active material is increased. Accordingly, when the pulse charging/discharging cycle is repeated for a long term, there has been a problem that the structural destruction of the positive electrode active material is caused according to over-voltage, which results in a large reduction in power of the battery.
In addition, in the case of the lithium-ion secondary battery, as the capacity and/or power thereof is increased, the safety thereof is apt to lower. Particularly, as mentioned above, in the non-aqueous electrolytic solution secondary battery for obtaining a high power (performance), such a tendency is observed that the phenomenon becomes intense when the battery falls in an abnormal state. That is, in reaction of the lithium-ion secondary battery at a time of over-charged state thereof, all lithium ions are released from the positive electrode active material and the structure of the active material is made unstable according to the over-voltage, so that the non-aqueous electrolytic solution becomes easy to decompose. When the non-aqueous electrolytic solution is decomposed, oxygen releasing reaction occurs according to the decomposition of the non-aqueous electrolytic solution in the boundary face where the lithium-manganese complex oxide and the non-aqueous electrolytic solution come in contact with each other. When the particle of the lithium-manganese complex oxide is small, since the area per unit volume of the boundary between the lithium-manganese complex oxide and the non-aqueous electrolytic solution increases and the oxygen releasing reaction increases, there is a problem that heat generation of the small particle due to the structural destruction thereof promotes the structural destruction of adjacent particles to cause heat generation due to chain structural destruction and to reach energy causing the structural destruction of the entire of the positive electrode active material so that heat generation of the battery and/or white smoke emission from the internal pressure releasing mechanism is caused.
In a case of such a high capacity and high power battery as used for power supply for an electric vehicle, because a large current charging and/or a large current discharging is caused at a normal using time of the battery, it is practically difficult to provide within the battery structure a current shutting-off mechanism (a kind of a shutting-off switch) actuated according to an increase in battery internal pressure at an abnormal time of battery, which is generally employed in the 18650 type lithium-ion secondary battery.
In a case of an electric vehicle for passengers, it is very important battery characteristic which is at least required that the safety of the battery itself is secured at a time of over-charging occurring in a case that a charging control system has broken down, at a time of battery crushing which may be caused in a case of accidental vehicle collision, at a time of foreign matter spitting, at a time of externally short-circuiting or the like. Incidentally, the safety of battery means not only that the behavior of the battery does not injure a person physically even when the battery has been put in an abnormal state but also that an damage to the vehicle is suppressed to the minimum range even in such a state.
In view of the above circumstances, a first object of the present invention is to provide a non-aqueous electrolytic solution secondary battery whose power characteristic is improved without enlarging the size of the battery. Also, a second object of the present invention is to provide a non-aqueous electrolytic solution secondary battery which has a high safety level while having high capacity, high energy density and high power (performance). Further, a third object of the present invention is to provide a non-aqueous electrolytic solution secondary battery which can maintain high power even when a pulse charging/discharging cycle is repeated for a long term.
In order to achieve the first object, the present invention is a non-aqueous electrolytic solution secondary battery where an electrode winding group where a positive electrode formed by applying positive electrode active material mixture including a lithium-manganese complex oxide comprising secondary particles formed of aggregated primary particles and conductive material on both surfaces of a strip-like collector by approximately even amounts to the both surfaces and a negative electrode from/in which lithium ions can be released/occluded through charging/discharging are wound through a separator is accommodated into a battery container having an internal pressure releasing mechanism which releases internal pressure at a predetermined internal pressure, wherein an average particle diameter (particle size) of the primary particles of the lithium-manganese complex oxide is in a range of from 0.1 xcexcm to 2 xcexcm.
In the present invention, when the lithium-manganese complex oxide where the average particle diameter of the primary particles is less than 0.1 xcexcm is used as the positive electrode active material, because the reaction area is increased but crystals in the material do not have grown sufficiently, the reaction resistance is increased and the power of the non-aqueous electrolytic solution secondary battery is lowered. On the other hand, when the lithium-manganese complex oxide where the average particle diameter of the primary particles is more than 2 xcexcm is used, because the reaction area is decreased and the current density per unit positive electrode active material becomes large, the power of the non-aqueous electrolyte solution secondary battery is lowered. Therefore, by using the lithium-manganese complex oxide where the average particle diameter of the primary particles is in the range of from 0.1 xcexcm to 2 xcexcm, the reaction area of the positive electrode active material is optimized so that the non-aqueous electrolytic solution secondary battery whose power characteristic is improved can be obtained without enlarging the size of the secondary battery.
At this time, by using the lithium-manganese complex oxide where a Li/Mn composition ratio is in the range of from 0.55 to 0.60, the amount of elution of manganese can be reduced without extremely lowering the discharging capacity of the secondary battery as compared with stoichiometric composition (Li/Mn=0.5). It is preferable that a complex oxide expressed by a chemical formula Li1+xMn2xe2x88x92xO4 or a complex oxide where a portion of manganese has been substituted for another metal element is used.
Also, in order to achieve the second object, the present invention is a non-aqueous electrolytic solution secondary battery according to the first aspect, wherein the amount of application of the lithium-manganese complex oxide per one side surface of the collector is in the range of from 80 g/m2 to 160 g/m2 and the amount of conductive material included in the positive electrode active material mixture is in the range of from 8 weight % to 16 weight %, or wherein the amount of application of the lithium-manganese complex oxide per one side surface of the collector is in the range of from 270 g/m2 to 330 g/m2 and the amount of conductive material included in the positive electrode active material is in the range of from 3 weight % to 7 weight %.
In a non-aqueous electrolytic solution secondary battery having high capacity and high power (performance), when it falls in an abnormal state, large current charging or large current discharging state is maintained so that a large amount of gas is generated rapidly due to chemical reaction between the non-aqueous electrolytic solution and the active material mixture within the battery container and internal pressure in the battery container is increased. In the non-aqueous electrolytic solution secondary battery, for preventing the internal pressure in the battery container from increasing, the battery container is generally provided with an internal pressure releasing mechanism. By employing such settings that the amount of application of the lithium-manganese complex oxide per one side surface of the collector is in the range of from 80 g/m2 to 160 g/m2 and the amount of conductive material included in the positive electrode active material is in the range of from 8 weight % to 16 weight %, or the amount of application of the lithium-manganese complex oxide per one side surface of the collector is in the range of from 270 g/m2 to 330 g/m2 and the amount of conductive material included in the positive electrode active material is in the range of from 3 weight % to 7 weight %, gas discharging from the internal pressure releasing mechanism is conducted remarkably gently. For this reason, according to the present invention, a non-aqueous electrolytic solution secondary battery which has a high safety while maintaining a high capacity and a high power can be realized.
In this case, by using mixture of graphite and amorphous carbon as the conductive material, a non-aqueous electrolytic solution secondary battery with higher power can be manufactured. At this time, when the average particle diameter of the graphite is in the range of from 0.2 to 0.8 times as large as the average particle diameter of the secondary particle and/or acetylene black is used as the amorphous carbon, a non-aqueous electrolytic solution secondary battery whose power is further increased can be obtained. Also, when the Li/Mn composition ratio in the lithium-manganese complex oxide is in the range of from 0.55 to 0.60, the power maintaining rate of the battery can be improved without causing reduction of the capacity thereof. Further, when amorphous carbon is used as the active material for the negative electrode, a non-aqueous electrolytic solution secondary battery with higher power, higher capacity and further excellent safety can be manufactured.
Further, in order to achieve the second object, such constitution can be employed that the lithium-manganese complex oxide which is expressed by a chemical formula Li1+xMn2xe2x88x92xO4 or where a portion of manganese in the chemical formula has been substituted for or doped with another metal element is used and the weight of the particles of the lithium-manganese complex oxide with a particle diameter of 1 xcexcm or less is 0.1% or less of the total weight of the lithium-manganese complex oxide. With this constitution, because the area per unit volume of the boundary between the lithium-manganese complex oxide and the non-aqueous electrolytic solution is reduced and heat generation due to oxygen elution reaction can be made small, a chain-like reaction between the positive electrode active material and the non-aqueous electrolytic solution can be suppressed. Accordingly, a non-aqueous electrolytic solution secondary battery which has an excellent safety while achieving a high energy density and a high power can be realized. In this case, when the specific surface of the positive electrode active material is set to 0.6 m2/g or less, because the area of the boundary between the positive electrode active material and the non-aqueous electrolytic solution can be reduced so that the chain reaction can be suppressed, a non-aqueous electrolytic solution secondary battery whose safety is further enhanced can be obtained.
In order to achieve the third object, the present invention is a non-aqueous electrolytic solution secondary battery wherein the lithium-manganese complex oxide which is expressed by a chemical formula Li1+xMn2xe2x88x92xO4 or where a portion of manganese in the chemical formula has been substituted with or doped with another metal element is used and the weight of the particles of the lithium-manganese complex oxide which have a particle diameter of 1 xcexcm or less is in the range of from 0.01% to 2%. In the present invention, by setting the weight of the particles with a particle diameter of 1 xcexcm or less to at most 0.01% of the total weight of the lithium-manganese complex oxide, space around respective particles with a particle diameter of 1 xcexcm or less is filled with the non-aqueous electrolytic solution and these particles are infiltrated with the non-aqueous electrolytic solution uniformly. For this reason, it is considered that, even when the pulse charging/discharging cycle is performed for a long term, because lithium ions are occluded into the positive electrode active material smoothly, the discharging reaction progresses easily and any voltage difference in the positive electrode active material does not occur so that a structural destruction does not occur. In this manner, such an effect can be expected that the particles having a particle diameter of 1 xcexcm or less reduce discharging load acting on the whole positive electrode. On the other hand, when the weight of the particles with a particle diameter of 1 xcexcm or less is 2% or more, binding force among the positive electrode active material particles is weakened and the positive electrode active material particles fall off from the positive electrode for a long term pulse charging/discharging cycle, which causes lowering of the power. Therefore, it is necessary to set the weight of the particles of the positive electrode active material having a particle diameter of 1 xcexcm or less to at most 2%. According to the present invention, by setting the weight of the particles of the positive electrode active material with a particle diameter of 1 xcexcm or less the range of from 0.01% to 2% of the total weight of the positive electrode active material, a non-aqueous electrolytic solution secondary battery which can maintain a high power even when the pulse charging/discharging cycle is repeated for a long term can be obtained. In this case, even when the specific surface area of the positive electrode active material is set to 0.6 m2/g or more, such an effect is expected that the reaction area increases and the discharging reaction progresses so that the discharging load acting on the whole positive electrode can be reduced. Meanwhile, when the specific surface area is made larger than 2.0 m2/g, the amount of elution of manganese ions retained at a high temperature increases, which makes current flow difficult, and the power is lowered due to the capacity reduction. Therefore, it is preferable that the specific surface area of the positive electrode active material is set to the range of from 0.6 m2/g to 2.0 m2/g.